1. Field of the Invention
The invention relates to an extruder head for extrusion blow-molding plastic containers, comprising                a ring-gap nozzle having a mandrel and a ring-shaped nozzle body;        an elastically deformable sleeve; and        setting devices for deforming the elastic sleeve radially.whereby the sleeve arranged on the outlet side of the nozzle defines a nozzle gap whose width is variable by axial setting movements of the mandrel and/or body of the nozzle, and whose geometry can be influenced by deforming the sleeve in the course of extrusion of a hose-like preformed blank; and whereby the sleeve is guided with radial mobility on sliding surfaces that support the sleeve upwards and downwards.        
2. Prior Art
An extrusion head with the features described above is known from DE-A 28 23 999 (FIG. 7). The sleeve, which is arranged in the body of the nozzle, has a symmetrical profile and a cylindrical center section as well as identical conical widenings at both ends of the sleeve. The upper and lower faces of the elastically deformable sleeve are guided on the body of the nozzle. The setting devices for deforming the sleeve radially are arranged in the plane of symmetry.
The pressure of the melt in the conical inlet zone of the sleeve amounts to multiple times the pressure of the melt prevailing in the conical outlet of the sleeve. The consequence thereof is that an axial force directed from the top down is applied to the sleeve. Said axial force is introduced into the body of the nozzle via the support on the underside. In addition, as the melt is flowing through the gap of the nozzle, a force directed downwards as well is acting on the sleeve because of the resistance to flow. Said force is dependent upon the through-put, the width of the annular gap, the viscosity, the temperature, as well as other factors. On its lower face side, the sleeve rests on the sliding surface of the nozzle body with only a narrow annular surface because any enlargement of the wall thickness of the sleeve is limited in that the latter is required to be deformable, on the one hand, and, furthermore, because the sleeve has to project radially inwards on the ring holding the body of the nozzle, on the other hand. Such inward projection is necessary in order to prevent the melt from running up on peripheral sections of the holding ring as it exits from the gap of the nozzle, said holding ring being a component of the nozzle. These constraints lead to the fact that the lower face side of the sleeve, which is supported on the body of the nozzle, is subjected to high molding pressure, and, furthermore, may be subjected to considerable wear during the radial movements occurring in conjunction with the deformation of the sleeve. Such wear is high because the sleeve is continuously subjected to radial movements due to the program-controlled motions of the setting devices occurring in the course of extrusion of the hose-shaped preformed blanks.